Electrical circuits, apparatus, and method



March 24, 1925. 1,530,633

.D. F. WHlTlNG ELECTRICAL CIRCUITS, APPARATUS, AND METHOD Filed Aug. 30.1920 /n ven for:

Dona/d E Whif/hy by My Patented Mar. 24, 1925.

UNITED STATES 1,530,633 PATENT OFFICE.

DONALD FAIRFAX WHITING, OF NEW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRICCOMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

ELECTRICAL CIRCUITS, APPARATUS, AND METHOD.

Application filed August 30, 1920. Serial No. 400,861.

An object of the invention is to provide 1 a potentiometer adapted to beadjustably connected to a frequency-responsive device and so arrangedthat the impedance of the portion of the potentiometer connected acrosssaid device is substantially the same 2 for all settings of thepotentiometer.

Another object of the invention is to pro vide a potentiometer which maybe economically constructed, and the method of making the same.

A further object of the invention is to provide a transformer which willhave a uniform transmission efiiciency over a wide range of frequencies.

Other objects of the invention will be clear from the following detaileddescription and.

claims, when taken in connection with the accompanying drawings in whichFig. 1 represents schematically a circuit embodying the invention andFig. 2 represents a slightly modified transformer.

Referring to the drawings by reference numerals, conductors 5 and 6which may, for example, be telephone line wires, are connected to theouter terminals of a po- 40 tentiometer 8 which is adjustably connectedto the primary 10 of transformer 11. The secondary12 of transformer 11is connected to the cathode 15 and grid or control electrode 16 of thevacuum tube amplifier 17.

A source of potential 18 is employed to maintain the rid negative withrespect to the cathode. athode 15 and anode 19 may be connected by meansof any usual or desired output circuit to the conductors 25 and 26,which may, for example, be the outgoing line wires.

Since the vacuum tube amplifier is a voltage operated device,transformer 11 is arranged to step up the voltage. It may, for

example, have an impedance step up ratio of 282 to 300,000 ohms. Theinput impedance of vacuum tube 17 with reference to direct current orlow frequencies is usually of the order of infinity and in any event ishigh relative to the impedance of the high winding of the transformer.At the higher frequencies of the voice range, however, the capacitybetween the tube electrodes, the capacity between the leads, and thecapacity within the coil itself are appreciable. For these reasons thetransformer presents an inductive reactance to the line in the lowerpart of the voice range and a capacity reactance in the upper part ofthe voice range. If these reactances are not annulled the repeater'maygive a desired gain in the middle portion of the frequency range, butthe transmission may fall off considerably at the upper and lower endsof the range, with resulting distortion of the speech signals. Thiswould also result in poor transmission of ringing currents, which inpractice usually have a low frequency, as for example, 135 cycles.

An inductance 30 is provided in series E with primary 10 having such avalue that it will resonate with the capacity reactance above describedat frequencies up to the upper end of the rangeof frequencies .which itis desired to transmit. As disclosed in Canadian Patent 226,477, Nov.21, 1.922, the inductive reactance employed to annul the capacityreactance may have the form of a separate inductance as shown in thedrawing, or it may be obtained by introducing leakage in the transformeritself. I

A condenser 32 is provided to annul the inductive reactance of thetransformer at frequencies in the lower part of the voice range. If thecondenser 32 were to be used alone in many cases the transmissioncharacteristic of the transformer would show a sharp upward curve at theresonant point, and it has been found desirable in some cases I toprovide means for damping the resonance. An arrangement in which aresistance is placed in series with the condenser for damping purposeshas been heretoforev shown. That arrangement, however, is not desirablein the circuit herein described, for the reason that a series resistancewould also cut down the currents at frequencies in the upper part of thevoicerange. It has been found, however, that a resistance 33 placed inparallel with condenser 32 will serve to flatten out the resonance curveof the low frequency tuned circuit, and at the same time a path of lowimpedance for currents of frequencies in the upper part of the voicerange will be provided through condenser 32. It is to be noted thatsince the secondary winding 12 is substantially opencircuited for lowfrequencies, the whole of the inductance of the primary winding 10 isavailable for tuning with condenser 32. It is thus possible to use asmaller condenser which is cheaper and has a higher impedance totelegraph currents of the order of 20 cycles. The potentiometer 8 ismade up of a plu rality of resistances 34 connected in series and havingtaps therebetween that may be selectively connected by means of acontact or contacts 36 to the primary 10. It has been found with thetypes of potentiometers heretofore employed that when the contactscorresponding to contacts 36 were adjusted to vary the repeater gain,thus varying the proportion of the resistance connected in series withthe primary 10, this so changed the constants of the circuit comprisingprimary 10 and associated elements that the transmission characteristicwas no longer uniform, even though the circuit had been adjusted to givea uniform transmission at a particular potentiometer setting. In orderto maintainthe impedance of this circuit constant, resistances 38 ofsuitably graduated values are connected between the potenti- .ometertaps and the connections between the series resistances 34. Theresistances in series with the extreme taps 39 are substantially zeroand the resistance values increase toward the center so that theresistance meas? ured between contacts 36 is always SllbStfl1l tiallythe same.

In order that the circuit may be balanced, primary 10 is divided andelements 30, 32 and 33 placed between the parts. point of potentiometer8 is preferably grounded at 40 to prevent. singing due to longitudinalcurrents.

As shown in Fig. 2, the resistance 42 is provided in series withinductance 30. This arrangement may be employed where it is desirable todamp the high frequency tuneid circuit. Resistance 42 will usually be oftoo small a value to damp the low frequency tuned circuit sufficiently.Resistance 33 is therefore present but will have a value greater thanits value when used as in Figure 1.

The ste )s of potentiometer 8 are usually made sma 1, representinglosses of from .2 of a mile to 2 miles of standard cable. If thesummation value of a plurality of resistances 34 is approximatelycorrect, a considerable variation in the values of the individualresistances is permissible. In some cases a variation of 40% in aresistance 34 will not seriously impair the operation of the gainadjusting device. It has been found that instead of accurately measuringthe resistance of each unit, the potentiometer may be much moreeconomically made by the following method: Resistance wire is cut upinto lengths each having a nominal resistance value calculated from thedimensions of the wire. These lengths may be wound on spools or formedinto units of any desired character. They are then thrown into boxeslabeled with the nominal values of the various sizes. The operatorselects a unit having the nominal value for the first step of thepotentiometer, and secures it to suitably mounted posts. She

then measures its actual resistance. If the actual value is within thepermissible variation the unit is not disturbed, but a second unit isselected having a nominal value greater or' smaller, as the case m. be,to compensate for the variation of t e first. In selecting the furtherunits the combined resistance of the units already attached is measuredand the variation from the desired value is compensated for in themanner just described. A potentiometer may thus be made in which each ofthe units varies from its nominal value, but the combined resistance ofany group of units will vary from its nominal valueby an amount lessthan the greatest variation of any individual of the group.

While the invention has been shown in connection with a vacuum tuberepeater, it is obvious that certain features of the invention arecapable of a variety of uses and that the invention is not to be limitedexcept as 'defined in the following claims.

What is claimed is:

1. A potentiometer, means for supplying electromotive force thereto, anda receiving circuit, said potentiometer having contacts for adjustableconnection.- to said circuit, and having means for rendering theimpedance, to current of a given nature, of said potentiometer andsource viewed as from said circuit substantially the samefor differentadjustments of said connection'of said contacts to said circuit.

2. In combination, a potentiometer, circuits connected to the fixed sideand adjustable side respectively of said potentiometer, saidpotentiometer presenting to said circuits, respectively, substantiallythe same impedance, to current of a given nature, in one adjustment ofsaid potentiometer as in another adjustment of said potentiometer.

3. In combination, a potentiometer, means for supplying electromotiveforce thereto, a receiving circuit, and means adjustably connecting saidcircuit to said potentiometer and maintaining the impedance, to currentof a given nature, measured across said circuit, substantially the samefor different adjustments of said connection of saidcircuit to saidpotentiometer.

4. In combination, a line, an impedance connected across said line, acircuit, and means whereby said circuit may be connected acrossdifferent percentages of said impedance, said means comprisingmeans forrendering the impedance looking from said circuit toward said fiistmentioned means-substantially the same for the different connec tions ofsaid circuit to said first mentioned impedance.

A potentiometer, means for supplying electromotive force thereto, anoutput circuit and means adj ustably connecting said output circuit tosaid potentiometer and maintaining the impedance to current of a givennature measured across said connections sub stantially the same at allpositions of adj ustment. I

6. A potentiometer comp-rising a resistance, a plurality of contacts,each havin a connection to a separate point on said reslstance,resistances in said connections, and a circuit connected to saidpotentiometer and having a terminal adapted to engage each of aplurality of said contacts, said resistances having values such that theimpedance of said circuit remains substantially constant at differentpositions of said terminal.

7. Iii combination, a line, a fixed resist-- ance connected thereacross,said resistance comprising a plurality of resistance elements connectedin series, contacts, resistances of different values connecting saidcontacts to a plurality, of points between said series elements, and acontact for traversing said first mentioned contacts.

8. A gain adjusting device variably con.- nected in the input circuit ofa repeater and means for maintaining the impedance of said circuitsubstantially constant when said de- Vice is varied.

9. A gain adjusting device variably connected in the input circuit of arepeater and means for maintaining the impedance of said circuitsubstantially constant when said device is varied, said means comprisinga plurality of resistances adapted to be separately connected in saidcircuit.

10. !In combination, a potentiometer, a frequency responsive deviceadjustably in circuit with said potentiometer, and means for maintainingthe impedance of said circuit substantially constant throughout aplurality of adjusted conditions.

11. In combination, a transformer, means altering the transmissioncharacteristics of said transformer for a certain frequency, apotentiometer adjustably connected to said transformer and means formaintaining the transmission characteristics of said transformerindependent of changes in the adjustment of said potentiometer.

transmission characteristics of said transformer independent of changesin the adju'stment of said potentiometer, said means .comprisin seriesresistances individual to the potentiometer contacts.

14:. The method of making a sectional impedance device, which comprisespreparing resistance elements whose values are only approximately known,connecting a plurality of said elements in succession in series,measuring at times the summation resistance of the elements alreadyconnected, and compensating for the variation of the measured value fromthe nominal value by selecting for the succeeding element one having theproper nominal value.

15. A. potentiometer, comprising resistance elements in series, saidpotentiometer being calibrated on the basis of a nominal value for eachof said resistances, each of a plurality of said elements having anactual value differing from its nominal value, but the true value of aseries group of said elements varying from the sum of nominal values ofthe elements constituting said group by an amount less than the greatestindividual variation among the elements of the group.

16. An electrical device comprising im pedanoe elements in series, saiddevice being calibrated on the basis of a nominal value for each of saidelements, each of a plurality of said elements having an actual valuediffering from its nominal value, but the actual value of a series groupof said elements varying from the sumv of the nominal values of theelements constituting said group by an. amount less than the greatestindividual variation among/the elements of the group. j

17. In combination, a vacuum tube repeater having cathode and controlelectrodes, a transformer having its secondary connected in series withthe s ace between said electrodes, a condenser orming with theinductance presented by the primary of said transformer a series tunedcircuit and a resistance in parallel with said condenser, said condenserand said inductanceresonating at v transformer a series tuned circuit, aresistance in parallel with said condenser, and an inductance in crieswith said parallel arrangement, said last mentioned inductance formingwith the capacity between the transformer windings and said electrodes acircuit which is resonant at frequencies for which the impedance of saidcondenser is negligible.

19. In combination, a transformer presenting in its primary circuit aninductive reactance at low frequencies'and a capacity reactance atrelatively high frequencies, a condenser and a resistance in parallelforming with said inductive reactance a circuit tuned to a relativelylow frequency, and an inductance forming with said capacity reactance acircuit tuned to a relatively high frequency.

20. In combination, a transformer presenting in its primary circuit aninductive reactance at low frequencies and a capacity reactance atrelatively high frequencies, a condenser and a resistance in parallelforming with said inductive reactance a circuit tuned to a relativelylow frequency, and an inductance forming with said capacity r'eactance acircuit tuned to arelatively high frequency, the primary of saidtransformer, said inductance and said parallel arrangement beingconnected in series.

21. In-combination, a transformer presenting in its primary circuit aninductive reactance at low frequencies and a capacity reactance atrelatively high frequencies, a

condenser and a resistance in parallel forming with said inductivereactance acircuit tuned to a relatively low frequency, and aninductance forming with said capacity reactance a circuit tuned to arelatively high frequency, and a second resistance for damping saidtuned circuits, the primary of said transformer, said inductance, saidsecond resistance and said parallel arrangement being connected inseries.

22. In combination, a vacuum tube repeater, an input transformertherefor, means resonating with the inductance and capacity of saidtransformer respectively, and a potentiometer adjustably connected tosaid the space between said electrodes, the primary of said transformerpresenting an inductive reactance at a frequency in the lower part ofthe voice range, and a condenser and a resistance in parallel connected1n series with said primary, said condenser being of such capacity as totune with said inductive reactance at said frequency in the lower partof the voice range, and said resistance being of such value as tomaintain the repeater gain at said resonance frequency substantially thesame as in the middle portion of the voice range.

24. An attenuation network having terminals for connection to a sourceof electromotive force and terminals for connection to a receivingcircuit, and adjusting means for varying the potential across the secondmentioned terminals for a given potential applied to said firstmentioned terminals while maintaining substantially constantlyimpedances as seen looking into both the first mentioned terminals andthe second mentioned terminals from without the net-work.

25. The method of operating a potentiometer having input terminals and areceiving circuit fed from said potentiometer which comprises alteringthe connection of the circuit to the potentiometer to impress differentelectrbmotive forces upon the circuit for a given electromotive forceacross said input terminals, and maintaining the impedance of thepotentiometer facing the circuit the same for the different connections.

In witness whereof, I hereunto subscribe my name this 24th day of AugustA. D. 1920.

DONALD FAIRFAX WI-IITIN G.

